This invention relates to a seal assembly for constant process ovens used in the manufacturing, such as the annealing, of ribbon or tape conductor used for superconducting magnets, and which allows the conductor to pass through the seal assembly while blocking out the surrounding atmosphere which would deleteriously affect the process.
As is well known, a magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnet coils to negligible levels, such that when a power source is initially connected to the coil (for a period, for example, of ten minutes) to introduce a current flow through the coils, the current will continue to flow through the coils due to the negligible resistance even after power is removed, thereby maintaining a magnetic field. Superconducting magnets find wide application, for example, in the field of magnetic resonance imaging.
Because of the extreme thermal and electromagnetic forces encountered by coils in superconducting magnets, unique ribbon or tape conductors are fabricated and used. The manufacturing process typically includes passing the tape conductor through a plurality of ovens during the manufacturing process, including, for example, coating, decomposition, and reaction anneal ovens. The manufacturing process is typically a constant process in which the tape is sequentially passed through the various manufacturing processing ovens. However, during such processing it is important that the air or atmosphere surrounding each oven, and in particular the oxygen in the air, be excluded from the ovens since oxygen could cause reactions which would deleteriously affect the ability of the tape conductor to meet the stringent requirements for superconducting magnet conductors.
An inert gas such as argon is typically maintained at a pressure within the oven to further insure against introduction of air. As a result, a seal is also required at the exit of each oven. The principal difficulty with such seals is that while the tape conductor must pass freely through the seals, the surrounding atmosphere must be blocked from passing through even in the presence of dross or other imperfections in the dimensions of the tape. Also, while a tight seal is required, the seal must exhibit resistance to wear and enlargement of the opening through which the tape conductor passes, which would then result in the loss of considerable inert gas from within the pressurized oven, and/or the introduction of oxygen into the oven. In a typical oven, such as a reaction anneal oven, the oven temperature is in the range of over 1000.degree. C.
Conventional seals after a period of use, exhibit wear and enable considerable leakage, resulting in the release of argon gas through the seals into the work area surrounding the ovens and the need to continuously replenish the argon gas within the oven.
As a result, it is important that seals for such ovens readily enable the tape conductor to pass through the oven, provide the best seal possible, and maintain sealing integrity over an extended period of use, even in the presence of wear through the continuous passage of the tape conductor through the seal. These requirements must be met even though a superconducting tape is a fragile foil in the order of 1 mil thick which acts somewhat like a continuous razor blade passing through the seal.
In addition, since such seals are utilized in continuous processing of superconducting tape conductors, it is important that the seals be readily and quickly replaceable to minimize manufacturing downtime when a seal replacement becomes necessary.